As a method for manufacturing a silicon single crystal used for manufacturing a semiconductor device, a Czochralski method (CZ method) for growing and at the same time pulling a silicon single crystal from a raw material melt in a quartz crucible is widely performed. In a CZ method, a seed crystal is dipped in a raw material melt (a silicon melt) in a quartz crucible in an inert gas atmosphere, and then both the quartz crucible and the seed crystal are rotated and at the same time the seed crystal is pulled so as to grow a silicon single crystal of a desired diameter.
In recent years, grown-in defect in a silicon wafer becomes a problem due to the development of high integration and the resulting miniaturization of semiconductor devices. Crystal defect is a factor degrading characteristics of semiconductor devices, and influences more increasingly with the development of miniaturization of a device. As such a grown-in defect, octahedral void-state defect, which is an aggregate of vacancies in the silicon single crystal produced by a CZ method (See Analysis of side-wall structure of grown-in twin-type octahedral defects in Czochralski silicon, Jpn. J. Appl. Phys. Vol. 37 (1998) p-p. 1667-1670), a dislocation cluster formed as an aggregate of interstitial silicon (See Evaluation of micro defects in as-grown silicon crystals, Mat. Res. Soc. Symp. Proc. Vol. 262 (1992) p-p 51-56) and the like are known.
It is shown that the introduced amount of each of these grown-in defects is determined by temperature gradient of the crystal on its growing interface and the growth rate of the silicon single crystal. (See the mechanism of swirl defects formation in silicon, Journal of Crystal growth, 1982, p-p 625-643.) As methods for manufacturing a low-defect silicon single crystal utilizing this principle, publication of Unexamined Japanese Patent Application No. H6-56588, for example, discloses a method slowing the growth rate of the silicon single crystal, while publication of Unexamined Japanese Patent Application No. H7-257991 discloses a method for pulling the silicon single crystal at a rate not exceeding the maximum pulling rate which is approximately proportional to the temperature gradient of the boundary region between the solid phase and the liquid phase of the silicon single crystal. Other methods such as an improved CZ method in which the temperature gradient (G) and growth rate (V) during the crystal growth are focused (See “Japanese Association for Crystal Growth”, vol. 25, No. 5, 1998) are also reported. It is thus necessary to control highly precisely the temperature gradient of the crystal.
In these methods, a structure (heat insulating member) for insulating radiant heat in a form of a cylinder or an inverted cone is provided around the silicon single crystal to be grown above the melt surface so as to control the temperature gradient of the crystal. Since the temperature gradient of the crystal at a high temperature of the crystal can be thereby increased, it is advantageous for obtaining a defect-free crystal at a high speed. In order to control accurately the temperature gradient of the crystal, however, the distance between the surface of the raw material melt and the lower end surface of the heat insulating member located above the surface of the raw material melt (hereinafter, sometimes referred to as DPM) is necessary to be controlled highly precisely to be a predetermined distance. It has been difficult with a conventional method, however, to control the DPM precisely such that the DPM is the predetermined distance.
In addition, as the crystal diameter increases, the location of the melt surface varies very much depending on the weight (varying thickness), deformation during operation and expansion of the quartz crucible, so that the location of the melt surface varies per batch of the crystal growth, which is a problem. Therefore, it becomes more difficult to control the interval between the melt surface and the heat insulating member precisely such that the interval is a predetermined interval.
In order to improve these problems, it is proposed in publication of Unexamined Japanese Patent Application No. H6-116083, for example, to provide a reference reflector in a CZ furnace and to measure a relative distance between a real image of the reference reflector and a mirror image of the reference reflector reflected on the melt surface so as to measure the distance between the reference reflector and the melt surface. This method is for precisely controlling the interval between the melt surface and the heat insulating member based on the measurement result such that the interval is a predetermined interval.
Furthermore, publication of Unexamined Japanese Patent Application No. 2001-342095 discloses a method in which curve of the raw material melt due to the rotation of the crucible is considered in order to obtain the stability of the mirror image of the reference reflector.
In these methods, the real image of the reference reflector and the mirror image of the reference reflector are captured by a detecting means such as an optical camera or the like. The brightness of the captured real and mirror images of the reference reflector is quantized to two levels (binarization process) by determining a constant threshold (threshold for binarization level). In other words, a brighter location and a darker location than the threshold for binarization level are distinguished. Then by measuring where the edge is located and by converting the measured value, the distance between the real image and the mirror image is measured.
However, there is a problem that the distance between the reference reflector and the melt surface cannot be stably and accurately measured since the brightness of the mirror image of the reference reflector reflected on the melt surface is changed over the time period of the crystal growth process and as a result a detection value by the optical camera before the binarization varies, or since a noise which is not a mirror image of the reference reflector such as a splash of melt attached to a structural part in the CZ furnace is detected.
As an another problem, if a raw material melt is contained in a quartz crucible having a bore diameter of 800 mm or more, and a silicon single crystal having a diameter of 300 mm or more is manufactured without applying a magnetic field, the melt surface is fluctuated, so that an accurate location of the melt surface cannot be stably detected. A relative distance between the reference reflector and the melt surface cannot be measured stably and accurately in this case either.
If the measuring result of the relative distance between the reference reflector and the melt surface is inaccurate, the interval between the melt surface and the heat insulating member cannot be controlled precisely to be a determined interval. As a result, a silicon single crystal with a desired quality cannot be manufactured with preferable productivity.